1. Field of the Invention
This invention relates to compatible single sideband (CSSB) transmission techniques, particularly with regard to the development of a CSSB signal within relatively narrow bandwidth restrictions, i.e. with precise spectral characteristics, and with low envelope distortion. In preferred form, such a system utilizes a three tone CSSB generator modulated with audio frequencies less than about one-half of the highest audio frequency signal acceptable within the bandwidth and modulates a SSBSC generator with the higher audio frequencies acceptable within the bandwidth, the generator outputs being summated to provide the transmitted signal.
2. Description of the Prior Art
Previous techniques for developing what is known as compatible single sideband modulation of an electromagnetic carrier wave are described in my prior U.S. Pat. Nos. 2,989,707; 3,012,209; 3,212,008; and 3,350,645. Also of interest are discussions of this general type of system as such appear in the National Association of Broadcasters Engineering Handbook, 1960, published by McGraw-hill & Company, at pages 8-41 through 8-52, and my article in Proceedings IRE, issue of October 1961, at pages 1503 through 1527. As referred to in said U.S. Pat. No. 2,989,707 at column 9, lines 40-47, and in said U.S. Pat. No. 3,212,008, at column 3, line 15, for example, a CSSB modulated carrier wave characteristically is a three tone wave at 100 percent modulation with a carrier level of about 0.7 (i.e. at a reduced level), a first order sideband level of about 1.0 (i.e. at full level), and a second order sideband level of about 0.3 (i.e. at a lower level). Further details as to the nature of my prior CSSB techniques may be had by making reference to the foregoing patents and articles and to the extent such are pertinent to the present invention the disclosure of said patents and articles are incorporated herein by this reference.
Of particular interest of the technique here presented involving CSSB modulation of the lower frequencies of the audio signal and SSB modulation for higher frequency components is early work in Great Britain and the Netherlands reported in an article by P. P. Eckersley, entitled "Asymmetric-Sideband Broadcasting",appearing in the September 1938 issue of Proc. IRE (Vol. 26), at pages 1041 -1092, and in an article by N. Kroomans entitled "Asymmetric-Sideband Broadcasting" appearing in the November 1939 issue of Proc. IRE. (Vol. 27), at pages 687-690. These articles report an asymmetric sideband broadcasting utilizing double-sideband transmission for low audio frequency components and single-sideband transmission for higher frequency components. These workers in the field recognized the fact that the high frequency components were generally low in amplitude and therefore distortion of full carrier single sideband transmission would not be as excessive; however, use of double sideband modulation for even the lower frequencies involves considerably more spectrum space than is required for a pure single sideband system or for the modulation system presented by this invention, in that the low frequencies in the double sideband modulation system require spectrum space in what might be termed the undesired sideband.
Also of general interest with regard to the subject compatible single sideband transmission systems are the following:
Olive U.S. 2,900,459 Olive 2,944,228 Logan Jr. et al 3,085,203 Kessel et al 3,274,492 Muller 3,277,376 Kessel 3,295,072 Remely 3,323,064 Kessel et al 3,378,772 Kessel 3,409,832 Kessel 3,449,715